1. Field of the Invention
This invention relates to a process for producing graphite blocks which have utility as X-ray and neutron ray monochromators, neutron ray filters and the like.
2. Description of the Prior Art
Graphite holds an important position as industrial materials because of its outstanding heat and chemical resistances, and high electric conductivity, and has been widely used as electrodes, heating elements and structural materials. Especially, single crystal graphite has good spectral and reflective characteristics and has been widely used as X-ray or neutron ray monochromators, filters or spectral crystal articles.
Natural graphite may be used for such purposes. Natural graphite with high quality occurs in an extremely limited amount and is intractable because of its powder or block form. Therefore, efforts of producing artificial graphite having such characteristics as natural single crystal graphite have been made. The production of such artificial graphite can be broadly classified into the following two processes.
In the first process, graphite is produced by separation from the melts of Fe, N/C system, decomposition of carbides of Si, Al and the like, or cooling of carbon melts under high temperature and high pressure. The graphite obtained by the process is called Kish graphite, and has the same properties as those of natural graphite. According to this process, however, only fine flakes of graphite are obtained. Therefore, together with the complexity of the manufacturing process and the expensive cost, this process has not been used in the industrial production.
The second process is one in which graphite is produced by pyrogenic deposition in a gas phase and hot working of gaseous hydrocarbons wherein re-annealing is effected at a temperature of 3400.degree. C. for a long time under pressure. Graphite thus obtained is called highly oriented pyrographite (HOPG) and has almost the same properties as those of natural graphite. This process enables one to produce graphite of very large sizes, unlike Kish graphite. This process has, however, the disadvantage that the manufacturing process is complicated with a low yield and the cost is very high.
In order to solve the problems involved in the above two processes and to produce graphite easily and inexpensively, graphitization of various organic matters or PG,4 carbonaceous materials by heating at temperature not lower than 3000.degree. C. has been attempted. In this process, graphite having the same characteristics as those of natural graphite or Kish graphite cannot be obtained.
Most of the polymeric materials cannot rather be used for this purpose. For example, the thermal treatment has been attempted to graphitize polymers such as phenolformaldehyde resins, polyparaphenylenes, polyparaphenylene oxides, polyvinyl chloride and the like. Since all of these polymers belong to a class of non-graphitizable materials, any product having a high degree of graphitization has not yet been obtained. For example, natural graphite and Kish graphite have an electric conductivity, which is the most typical property of graphite, of from 1.times.10.sup.4 S/cm to 2.5.times.10.sup.4 S/cm. In contrast, only the product having 1.times.10.sup.3 S/cm to 2.times.10.sup.3 S/cm can generally be obtained by this process. This indicates that graphitization does not well proceed in this process.
We extensively made studies to solve the problems of the manufacturing process of graphite from such polymers as set out above wherein a number of polymers were used for graphitization. As a result, it was found that films of polymers such as an aromatic polyamide (PA), a polyoxadiazole (POD), an aromatic polyimide (PI), three polybenzobisthiazoles (PBBT), a polybenzooxazole (PBO), a polybenzobisoxazole (PBBO), a polythiazole and the like could be easily graphitized. Based on this finding, we proposed a graphitization process in European Patent Application No. 0 205 970.
According to this process, graphite having a high degree of graphitization can be obtained easily within a short time by heating the polymers indicated above to temperatures not lower than 1800.degree. C., preferably not lower than 2500.degree. C.
The degree of graphitization is often expressed by X-ray diffraction parameters such as a lattice constant and a crystallite size in the direction of c axis, or by a rate of graphitization calculated therefrom, along with electric conductivity. The lattice constant is calculated from the (002) refraction line of X-ray, and a value nearer to the lattice constant of natural single crystal graphite of 6.708 angstroms indicates a more developed structure of graphite. The crystallite size in the direction of c axis is calculated from the half-width value of the (002) refraction line. A larger crystallite size indicates a more development of the plain structure of graphite. The crystallite size of natural single crystal graphite is 1000 angstroms or over. The degree or rate of graphitization is calculated from face spacing (d.sub.022) (Les Carbons Vol. 1, p. 129, 1965). As a matter of course, the degree of graphitization of natural single crystal graphite is 100%. The electric conductivity is a value determined along the direction of ab plane of graphite wherein a larger electric conductivity shows a more resemblance to the graphite structure. Natural single crystal graphite has an electric conductivity of 1.times.10.sup.4 to 2.5.times.10.sup.4 S/cm.
Another X-ray diffraction parameter used to evaluate the graphite structure is a rocking characteristic showing the manner of superposition of the ab planes. This is called a diffraction intensity curve which is obtained by rotating the crystal when monochromatic parallel X-ray fluxes are passed and measuring by fixing the value of 2 .theta. at the angle where the (00 l) diffraction line appears and rotating the angle, .theta.. This value is evaluated by the half-width value of the absorption and expressed by the angle of rotation (.degree.). A smaller value shows more clearly superposed ab planes.
The process of producing graphite from afore-described polymer films of the specific type is a very good process since it is simple and inexpensive. However, further studies revealed that this process had several drawbacks.
The first problem is that thick graphite blocks cannot be produced according to the process. The graphitization reaction may be considered to be apparently irrespective of the thickness of the starting film. In fact, this reaction depends strongly on the thickness of starting film. This has not been known but we experimentally confirmed the above fact. For instance, Table 1 shows the results of a test where four films of a polyoxadiazole with different thicknesses were graphitized to determine the lattice constant, degree of graphitization and electric conductivity along the ab planes of the resultant graphite.
TABLE 1 ______________________________________ Thickness of Treating Degree of Electric Starting POD Temper- Lattice Polymer- Conduct- Film ature Constant ization ivity (.mu.m) (.degree.C.) (angstroms) (%) (S/cm) ______________________________________ 5 2600 6.710 99 9800 25 2600 6.713 97 7800 100 2600 6.720 94 6100 450 2600 6.731 87 4900 ______________________________________
The above results demonstrate that the thickness of the POD film apparently influences how the graphitization reaction proceeds. For example, the degree of the graphitization varies from 99 to 87%, depending on the thickness of the film. This reveals that a thin film of graphite is obtained, but a thick graphite block is difficult to obtain.
The second problem of the prior art process is that mere heating of polymer materials does not improve the rocking characteristic.
The rocking characteristic is an important characteristic when graphite crystals are used as an X-ray optical element although the thickness and rocking characteristic of graphite crystal depend on the type of radiation optical element as will be described hereinafter. With the POD graphite indicated in Table 1, the rocking characteristic is 6.7.degree. for the starting film thickness of 5 .mu.m, 10.5.degree. for the starting film thickness of 25 .mu.m, 12.degree. for the starting film thickness of 100 .mu.m, and 17.degree. for the thickness of 400 .mu.m. Thus, the rocking characteristic is far from satisfactory. Thick radiation optical elements having a good rocking characteristic cannot be obtained by mere heating of polymer films for graphitization. It is considered that the cause for the poor rocking characteristic resides in a difficulty in orientation of the ab planes because a greater film thickness permit gases to generate in larger amount from the inside of the film undergoing the thermal treatment.